The <key> declaration, along with the <keyref> declaration, enables you to define a relationship between
two elements. The key/keyref mechanism functions similarly to database keys or to the ID/IDREF mechanism
built into XML DTDs. For example, an element might contain a <key> that is unique within a specified
context or scope. Another element can refer to the key using a <keyref> element. A <key> is always
defined inside an <element> declaration. It contains a <selector> element that defines the context or
scope of the key, and a <field> element that defines the specific key node. Like other elements, it can also
contain an <annotation>